Field of the Invention
The present invention relates to a semiconductor light emitting device. More particularly, this invention relates to a super-luminescent diode.
Description of the Related Art
Super-luminescent diodes (to be referred to as SLDs hereinafter) have been attracting attention in recent years.
That a semiconductor laser can be made to emit highly coherent light having a very narrow spectral half width with high output power by causing amplified stimulated emission light to resonate and also causing the emitted light to oscillate by means of a low injection current and that a light emitting diode (LED) can be made to emit light with a wide angle of radiation by utilizing spontaneous emission, are well known. Unlike those devices, SLDs are characterized in that they emit light having a broad spectral half width with high output power by using an arrangement of not causing the emitted light to resonate in a condition of high current injection, although they involve stimulated amplification.
Light that is stimulated and amplified so as not to resonate in a condition of high current injection in this way will be referred to as “SLD light” hereinafter in this letter of specification of this invention. Note that light emitted from the same region of an active layer includes both a spontaneous emission light component and an SLD light component.
The field of SLD applications has been expanding to include spectroscopes, length measuring devices, refractive index distribution measuring devices, tomography apparatus, excitation light sources and other instruments. For example, in the field of medical applications, devices referred to as optical coherence tomographic image measuring devices or optical coherence tomography apparatus (to be abbreviated as OCT apparatus hereinafter) are already known. OCT apparatus require a broadband and low coherent light source. More specifically, fundus examination OCT apparatus require a light source having a center wavelength of about 850 nm or 1,060 nm and representing an emission spectral shape that is close to Gaussian curve.
When an SLD is employed in a device of any of the above-described types and the optical output and the spectral shape thereof are to be controlled, conceivable techniques for exploiting SLD light for the intended purpose may include a technique of partly branching the SLD light to be used by means of an optical fiber or the like and a technique of receiving SLD light emitted from the light emitting surface thereof located opposite to the light emitting side of the SLD to be utilized for the intended purpose. Then, with either of the above-described techniques, the results obtained by analyzing the spectral distribution of the branched light or the received light are utilized to control the optical output and the spectral shape.
Specific exemplar techniques of controlling SLD light will now be described below by referring to FIGS. 3A and 3B. FIG. 3A schematically illustrates an arrangement for branching SLD light 806 emitted from an SLD device 805 having a single ridge type upper electrode for current injection, namely an SLD device having a single electrode structure, by means of a branching mirror 802. Note that FIG. 3A illustrates a plan view of the SLD device 805 as viewed from the side of the upper electrode 801.
The emitted SLD light 806 is divided by the branching mirror 802 into SLD light 803 to be used for the intended purpose and SLD light 804 to be entered into a detector 811. The arrangement is also so designed as to adjust the electric current being injected into the upper electrode 801 according to the detection signal generated by the detector 811. Thus, as a result of this arrangement, the optical output and the spectral shape of the SLD device 805, among others, are controlled.
FIG. 3B, on the other hand, schematically illustrates an arrangement for detecting SLD light 807 emitted from the edge face opposite to the light emitting side of the SLD device for emitting SLD light 806 to be used for the intended purpose by means of a detector 812 and adjusting the current injection amount into the upper electrode 801 according to the detection signal generated by the detector 812. Thus, as a result this arrangement, the optical output and the spectral shape of the SLD device, among others, are controlled.
Apart from the arrangements illustrated in FIGS. 3A and 3B, Japanese Patent Application Laid-Open No. 2011-66138 (to be referred to as Patent Literature 1 hereinafter) describes an arrangement for monitoring the reflected light from an edge face of an SLD device and adjusting the electric current being injected into the electrode of the SLD device according to the monitored value. Additionally, Japanese Patent Application Laid-Open No. H06-53546 (to be referred to as Patent Literature 2 hereinafter) describes an arrangement for monitoring the output of an SLD device from the second region thereof and adjusting the electric current being injected into the first region according to the monitored value.
However, while not only the optical output but also the spectral shape need to be controlled for SLDs that are required to represent spectral characteristics including a broadband spectrum and a spectral shape that is close to Gaussian curve, no conventional technique can control both of them in a simple and easy manner.
For example, as a conceivable technique for checking the spectral shape of an SLD device, the SLD light emitted from the device may be branched in a manner as illustrated in FIG. 3A and the branched light may be measured by means of an optical spectrum analyzer. However, such a technique is accompanied by a problem of requiring the use of a bulky arrangement, which entrails high cost and inevitably reduces the optical output that can be used for the intended purpose.
Furthermore, in the case of techniques of detecting (the optical output of) light emitted from only either one of the edge faces of an SLD such as the techniques illustrated in FIGS. 3A and 3B and those described in Patent Literatures 1 and 2, information that is detectable by any of those techniques is only the optical output of SLD light unless an optical spectrum analyzer or a similar instrument is additionally employed. In other words, with any of those techniques, information that accurately reflects changes in the beam characteristics of SLD light can hardly be obtained.
Particularly, when light emitted from the first edge face and light emitted from the second edge face of an SLD represent different beam characteristics and the emitted light monitoring arrangement is designed to monitor only light emitted from either of the edge faces, the optical output and the spectral shape cannot be controlled simply by adjusting the current injection amount unless the relationship between the beam characteristics of the light emitted from the first edge face and those of the light emitted from the second edge face is predetermined and known in advance. Additionally, when more than one electrodes are provided for current injection, there arises a problem of obtaining information necessary for individually controlling the injection current of each of the electrodes.
Note that the expression of the beam characteristics of emitted light as used herein includes the mean value or the change with time of the output power of light emitted from an SLD (SLD light in particular), the spectrum, the radiation angle and the polarization direction.
In order to obtain a broadband spectral shape for SLD light, for example, both light emitted from the ground level and light emitted from excited levels of the active layer may sometimes be employed in combination. An SLD having such a feature will be described below by referring to FIG. 6.
Referring to FIG. 6, the horizontal axis indicates the wavelength and the vertical axis indicates the amount of light. The plurality of curves drawn in FIG. 6 represents different carrier injection densities. The overall amount of light is small when the injection density is low but the overall amount of light tends to increase as the injection density rises. FIG. 6 also illustrates that the spectral shape changes to a large extent as a function of injection density and that an increase in the amount of light is observable as a function of increase of the injection density particularly at the short wavelength band side.
In FIG. 6, the straight line 1001 indicates the wavelength of light emitted from the ground level and the dotted line 1002 indicates the wavelength of light emitted from an excited level.
As the current injection amount is increased, the intensity of light emitted from the ground level increases in a low current injection range but the growth of the intensity of light emitted from the ground level becomes low, whereas the intensity of light emitted from an excited level begins to grow remarkably in a high current injection range.
Thus, changes in the spectral shape can hardly be grasped if only the overall optical output is monitored. In view of the above-identified problem of the prior art, therefore the object of the present invention is to provide an arrangement for a light emitting device, which may particularly be an SLD that is required to represent a broadband spectral shape, that enables the optical output and the spectral shape thereof to be easily, accurately and reliably controlled in a short period of time, a method of controlling an SLD having such an arrangement and also an optical coherence tomography apparatus using such an SLD.